Conventionally, it has been sometimes required that a plurality of motors be rotated at the same rotational speed. FIG. 5 is a block diagram showing the configuration of a publicly worked air blow system that provides the same quantity of air from a plurality of fans. Because n fans F1, F2, . . . Fn are designed in accordance with the same specifications, motors M1, M2, . . . Mn for rotating the respective fans are set to be rotated at the same rotational speed.
The motors M1, M2, . . . Mn are driven by motor drivers D1, D2, . . . Dn respectively. The motor drivers D1, D2, . . . Dn supply driving voltages V1, V2, . . . Vn to the motors M1, M2, . . . Mn respectively. The rotational speeds T1, T2, . . . Tn of the motors M1, M2, . . . Mn are detected by rotational speed detectors P1, P2, . . . Pn respectively and sent to the motor drivers D1, D2, . . . Dn.
Motor drivers Di (i=1, 2, . . . , n) each include a resistor R0i. In each of the motor drivers Di, a voltage across the resistor R0i is input to an operation unit CPU which determines a driving voltage Vi based on this voltage and allows a driving circuit DRV to supply the driving voltage Vi to a motor Mi.
The voltages across the resistors R0i depend on a current I supplied from a remote controller CO and the number n of the motor drivers Di. The motor drivers Di are all designed in accordance with the same specifications, and ideally an equation (1) holds:R01=R02=. . . =R0n=R0  (1)
Accordingly, when the motor drivers D1, D2, . . . Dn are connected in parallel with respect to the remote controller CO as shown in FIG. 5, based on the assumption that the input impedance of the operation unit CPU is sufficiently high, currents I1, I2, . . . , In flowing through the resistors R01, R02, . . . , R0n respectively are equal to each other, and an equation (2) holds:I1=I2=. . . =In=I/n  (2)
Because the motor drivers Di are all designed in accordance with the same specifications, an equation (3) holds for the driving voltages:V1=V2=. . . =Vn  (3)
Thus, when the motors M1, M2, . . . Mn are designed in accordance with the same specifications, the rotational speeds T1, T2, . . . Tn are equal to each other, whereby the same quantity of air is provided from the n fans F1, F2, . . . Fn.
The rotational speeds T1, T2, . . . Tn are adjusted by adjusting the magnitude of the current I supplied from the remote controller CO. For this reason, the remote controller CO is provided with a variable resistor Rv for current value setting.
As described above, in the conventional technique, the current I is supplied from the remote controller CO and the magnitude of the current I is adjusted to thereby adjust the same quantity of air provided from the n fans F1, F2, . . . Fn. Therefore, the rotational speed fluctuates only slightly even when the distance between the remote controller CO and the motor drivers D1, D2, . . . Dn increases which leads to a lot of resistance in the wiring length between them. It is also resistant to extraneous noise.
However, as is evident from the equation (2), a current Ii that is supplied to each of the motor drivers Di and generates the voltage across the resistor R0i depends on the number n of the motor drivers Di. Therefore, a voltage input to the operation unit CPU in each of the motor drivers Di varies when the number n of the motor drivers varies.
On the other hand, because the single remote controller CO is connected to a plurality of motor drivers, the current I output from itself and the same quantity of air provided from a plurality of fans are set in one relationship without consideration of the number n. Therefore, in order to obtain the quantity of air in accordance with the relationship set at the remote controller CO even when the number n varies, it is necessary to reset the relationship between the voltage across the resistor R0i and the driving voltage Vi at each of the motor drivers Di.
Besides, although the equation (1) holds ideally, the values of the resistors R0i sometimes vary among the motor drivers Di. In that case, the current Ii supplied to each of the motor drivers Di also varies, which in turn may cause rotational speeds Ti to vary. And in that case, the quantities of air provided from the fans F1, F2, . . . Fn will vary as well.